Evolution of carbonated lacustrine environment with stromatolites : a paleoecological approach (quarry of Montaigu-le-Blin, Limagne graben, Allier, France)

La carrière de Montaigu-le-Blin, située en Limagne bourbonnaise permet d’analyser en détail la sédimentation lacustre aquitanienne et de reconstituer les environnements de dépôt. Les sédiments marneux et calcaires renferment une faune d’eau douce et une flore particulièrement développées. Les dépôts sédimentaires traduisent des variations périodiques du niveau de la tranche d’eau qui provoquent aussi des variations périodiques des conditions chimiques au sein du lac. Ils sont représentatifs de milieux alternativement anoxiques et oxygénés. Ces variations sont le résultat d’alternances de périodes d’humidité variable, et seraient donc d’origine climatique. Ces alternances ont provoqué des variations périodiques des assemblages floro-fauniques : des thanatocœnoses surviennent durant les périodes anoxiques, alors que le développement majeur des algues encroûtantes responsables de la formation de concrétions algaires, apparaît lors des périodes oxygénées. Ces périodes sont également marquées par une augmentation de l’hydrodynamisme, associée à des apports détritiques du bassin versant. Les stromatolithes présents dans le milieu montrent des morphologies très variées, tributaires des associations de flore et de faune (algues, bactéries, fourreaux de larves de Trichoptères) qui les composent, ainsi que de leur milieu de croissance. Les associations de stromatolithes forment des complexes plurimétriques que l’on propose de situer par rapport à un paléorivage

direct detection and identification of uranium(vi)-bearing solids by trlfs and chemometrics analysis

International audienceUranium can be naturally found in the environment in the form of minerals, precipitates, and/or associated to other compounds like iron (hydr)oxides or clay rocks. Identification of the nature of the uranium compounds is of major importance for geological survey, mining exploration, and management of the environmental impact of industrial sites. Many studies have been conducted to characterize uranium-bearing solids with low uranium content from ppm to hundreds of ppm. This requires a careful search of uranium-rich zones in the sample, and the use of characterization methods such as SEM, XRD, and total elemental analysis. Such a methodology is rather long and difficult, and limits the survey to a small number of samples.Time-resolved laser induced fluorescence spectroscopy (TRLFS) is a classical technique that enables the detection of U(VI) at low level, and has been largely used for quantitative and speciation analyses, in solution [1] and in minerals [2]. The rapidity of the analysis is particularly interesting to investigate a large number of samples with no or limited preparation steps, to detect traces of U(VI) and to obtain spectroscopic information that can be related to the nature of the uranium(VI)-bearing phases such as phosphates. In this study, about twenty U(VI) fluorescence spectra have been recorded on a large set of samples from several environmental sites. The fluorescence intensity is however highly variable regarding the phases due to enhancing and quenching effects on the fluorescence. Interpretation of these measurements is not straightforward. Measurements on cooled or cryogenized samples can considerably increase the fluorescence intensity and the resolution of the spectra, and help identifying the chemical family of the phases [3]. However the resulting spectral analysis is often ambiguous for phase identification and improved data treatment methods need to be developed.Chemometrics provide useful multivariate methods for spectral and classification analysis, but is seldom used in TRLFS data analysis. We have applied multivariate methods to a set of U(VI) minerals and synthetic phases (phosphates, vanadates, arsenates, silicates) as reference materials. Independent Components Analysis (ICA) has been used to determine pure independent signals contributing to the fluorescence spectra. The resulting spectral signatures have been interpreted regarding the nature of the phases, and may evidence mixtures of compounds or U(VI) chemical sites in natural minerals. Using ICA, the spectral data sets have been categorized into classes that were successfully described using Principal Components Analysis (PCA). Iterative PCA has proved a good efficiency to discriminate most of the samples in our reference data set (Figure 1). Several classes cannot be easily distinguished, and exhibit a significant variability, which may suggest less pure compounds. The use of predictive methods with a dedicated spectral data base is under development in order to identify U(VI) phases in unknown samples. This combined TRLFS and chemometrics approaches has already demonstrated its great performance for U(VI) phases identification, and perspectives will be presented

Upper Cretaceous feldspars in the Cenozoic Limagne Basin:A key argument in reconstructing the palaeocover of the Massif Central (France)

International audienceThe northern part of the N–S-trending basin of the Limagne graben, located in the northern part of the Massif Central (France), contains Oligocene and Miocene calcareous lacustrine deposits. The area is known for its stromatolite reefs surrounded by marl, clay and uncemented carbonate sand with oncolites. Because the origin of the carbonate accumulated by cyanobacteria has long been debated, the clayey deposits associated with the stromatolites were analysed using XRD, SEM and K–Ar dating methods. The results show a monotonous assemblage of carbonates, dioctahedral smectite, illite–smectite (Reichweite) R=0, glauconitic illite and abundant orthoclase. Detrital minerals such as quartz and detrital illite are found near the basin's borders but are very scarce in its central part, indicating a lack of detrital input from the regional basement. Hence, the abundant orthoclase found in all the clayey deposits of the Limagne Basin cannot be linked to erosion of the surrounding Hercynian basement. The SEM analysis showed the orthoclase crystals to have a tabular morphology with sharp boundaries and to be embedded within a clay matrix, whilst the K–Ar dating of the orthoclase and glauconitic illite yielded ages ranging from 90 to 66±2 Ma, i.e. Turonian to Maastrichtian. These two results clearly indicate that the fill of the Limagne Basin, composed mainly of carbonate, clay and orthoclase, must have been partly due to the erosion of an Upper Cretaceous sedimentary blanket once covering the Massif Central basement. This cover was probably constituted of chalk, dioctahedral smectite, I/S and glauconitic illite, associated with authigenic orthoclase and flint. Flints are found reworked in the alluvial formations of the Late Pliocene Lower Bourbonnais sands and clays (‘Sables et argiles du Bourbonnais') and in the Pleistocene terraces. Moreover, relicts of the Upper Cretaceous cover were preserved during the sedimentary filling of the Cenozoic Limagne Basin. Authigenic orthoclase of similar shape has been described in the Chalk Group of the Paris Basin and from localities in northern France and Belgium. The mineral is rare in the Turonian chalk but increases rapidly at the base of the Coniacian to Campanian chalk; it has also been noted that the number of orthoclase crystals increases as the quantity of clastic components decreases. Similar orthoclase crystals were found associated with dioctahedral smectite in the decarbonated residues of the Coniacian to Santonian chalk from Fécamp and Etretat (northern France). Moreover, our results are consistent with other data from within and around the Paris Basin, such as the palaeogeography and facies distribution of the Chalk formations, the residual flints in the clay-with-flints of the southern Paris Basin and around the Morvan, and apatite fission-track thermochronology data from the Hercynian basement of the Massif Central and the Morvan. All these data indicate a major connection between the Paris Basin and the Tethys, and an extensive palaeocover on the Massif Central and Morvan basement